Electrical potentiometer and method of making same



April 4, 1961 BELL 2,978,662

ELECTRICAL POTENTIOMETER AND METHOD OF MAKING SAME Filed Nov. 26, 1958 2 Sheets-Sheet 1 A 3O 4 42 26 r k 2 f V 0 45 fi I ;y' DONALD 6. BELL INVENTOR,

BY m @W April 4, 1961 BELL 2,978,662

ELECTRICAL POTENTIOMETER AND METHOD OF MAKING SAME Filed Nov. 26, 1-958 2 Sheets-Sheet 2 l 2s 32 1 y' DONALD 6. BELL INVENTOR,

BY WM 2,978,662 lCfi ram ed Apr. 4, 1 961" ELECTRICAL POTENTIOMETER AND METHOD OF MAKING SAME Donald G. Bell, Covina, Camera and Instrument Delaware Calif., assign'or to Fairchild This invention pertains generally to electrical potentiometers, and more particularly to electrical potentiometers having helical resistance elements, as well as to a method of assembly of such potentiometers.

One common form of helical potentiometer generally includes a helical resistance element having a central axis, a rotary shaft positioned on such axis and an electrical contact mounted on such shaft for rotation therewith and for linear translation relative thereto alongvthe length thereof. The contact engages a portion of a single turn of the helical resistance element at a given instant, and is constrained to slide along the inner surface of the helix as the shaft is rotated. Upon the occurrence of such rotation, the electrical contact is rotated about the central axis of the helix, with the result that the contact itself traverses a helical path as it is moved from one point to another on the helical resistance element. It is generally preferable to provide a cylindrical outer casing for the unit, the end plates of which casing provide the necessary bushings for mounting the opposite ends of the rotary shaft. Terminals are provided for the main resistance element at spaced-apart points thereon, and a third terminal is connected to the movable contact.

In order to support the several turns of the helical resistance element within the cylindrical casing and to insulate the turns both from each other and from the casing (which is usually metal), suitable insulating means are generally provided between the helical resistance element and the casing, with helical grooves on the inner surface of the insulating material supporting the several turns of the helix. The prior art affords many examples of such insulating members between the turns of the helical resistance element and the metallic outer casing, but in most instances the devices of the prior art are unnecessarily cumbersome and expensive to manufacture.

It is accordingly a primary object of the present invention to provide an electrical potentiometer having an improved means for insulating the several turns of the helical resistance element thereof from each other and from an outer casing therefor.

Another object of the present invention is to provide an improved method of assembly of a helical potentiometer device.

Yet another object of the present invention is to provide an improved insulating sleeve for supporting the turns of a helical resistance element in an electrical potentiometer whereby the several turns are insulated both from each other and from the outer casing of the potentiometer.

In accordance with the present invention, the above and other objects are achieved by means of a pair of semicylindrical insulating sleeves the concave surface of each of which has a plurality of semi-helical grooves therein each supporting a portion of a turn of the helical resistance element of an electrical potentiometer. The semicylindrical sleeves are substantially identical as to the number, pitch and positioning of the grooves thereon, and are disposed in diametrically opposed relationship about the Corporation, a corporation of central axis of the helical resistance element. In order to provide a substantially helical support path for the helical resistance element, the two sleeves are relatively displaced along the aforementioned axis a distance equal to one half of the pitch of the grooves in the sleeves, whereby the ends of the grooves in one of the sleeves are substantially aligned with corresponding ends of respective grooves in the other sleeve. In a preferred form of the device of the present invention, each of the sleeves is somewhat less than a half of a cylinder, with the portions of the helical resistance element supported by the several grooves each being correspondingly less than a half turn of the resistance element, whereby the support path for the resistance element is an interrupted helix.

With the above considerations and objects in mind, the invention itself will now be described in connection with a preferred embodiment thereof given by way of example and not of limitation, and with reference to the accompanying drawings, in which:

Fig. 1 is a side elevation of the electrical potentiometer of the present invention, with portions being broken away for clarity of description.

Fig. 2 is an end view in elevation of the electrical potentiometer of Fig. 1, With the near end plate removed to show the several elements lying within the outer casing thereof.

Fig. 3 is a side elevation of the insulating sleeves of the present invention, shown in positions corresponding to those of Fig. l, and with a portion being broken away for clarity of description.

Fig. 4 is a plan view of the lower insulating sleeve of Fig. 3.

Referring now particularly to Fig. l, the electrical potentiometer of the present invention is shown in its assembled form therein. A cylindrical outer casing 10, usually of a suitably strong metal, is provided with a pair of end' caps 12 and 14 which are clamped to the casing 10 by means of respective annular clamping means 16 and 18.

The end caps provide a means for sealing the inside of casing 10 against moisture and dirt, and also serve as support plates for a rotary shaft 20 which extends through the casing 10 along the axis thereof, and which is mounted for rotation about such axis by means of bushings or the like (not shown) in each of the end caps 12 and 14.

Contiguous with the inner surface of the casing 10 is an insulating lining comprising a pair of semi-cylindrical sleeves 22 and 24, each of a suitable insulating material such as a moldable plastic, relatively disposed in diametrically opposed relationship about the central shaft 20. Each of the sleeves 22 and 24 is somewhat less than a half of a cylinder, so that a small space exists between the adjacent edges of the two sleeves as shown in Fig. 1 and Fig. 2. A helical resistance element 26 is supported by the inner or concave surface of the sleeves 22 and 24,

with the several turns of the resistance element 26 lying in respective grooves 28 in such surface.

A helical conductor member 30 is also supported by the grooved inner surfaces of the two sleeves 22 and 24, each turn of the resistance element 26 lying between a respective pair of adjacent turns of the helical conductor member 30. Conductor member 39 serves as a slip-ring in a manner that will be further explained in connection with subsequent figures of the drawings herein, and is supported by the sleeves 22 and 24 in the grooves 32 which lie between the resistance element grooves 28. A terminal 34 is connected to the conductor member 30, and constitutes the variably positioned tap of the potentiometer.

A pair of terminals 36 and 38 are connected to the resistance element 26 near the opposite ends thereof, and a third terminal 40 is connected to some convenient point along the length of such element. It will be appreciated amass by those skilled in the art that the resistance element 26 may be of any type suitable for use in the helical form shown. While the element may thus be a helix of some suitable resistance material, it is generally more desirable to provide a helical support member (which may, for example, be a suitable insulating material) around which a resistance wire is wound in helical fashion. Thus the resistance wire itself is in the form of a helix the center line of which is also a helix.

As may better be seen in Fig. 2, the resistance element 26 and the conductor. member 36 are substantially coaxial helices, with rotary shaft lying on the common axis. Shaft 20 is provided with a non-circular guide member 42 which is secured to the shaft and which provides a linear path along which a contact support 44 is free to slide in a direction substantially parallel to the shaft A contact 46 carried by the support member 44 slidably engages a variably selectable portion of a single turn of the resistance element 26, while a contact 48 similarly engages a portion of a single turn of the conductor member 39. Contact 48 may conveniently carry a tab or car 49 in order to maintain engagement of the contact on the inner periphery of the conductor member 3t as shaft 20 is rotated. Upon such rotation, contact 48 not only provides a continuous conductive connection with the slip-ring conductor member 30, but acts as a mechanical follower to produce linear translation of the sliding contact supporting member 44 along the guide member 42 as the contact 48 slides along the helical path defined by the conductor member 30. Since contacts 46 and 48 are connected together in a conductive relationship, and since terminal 34 is connected to the slip-ring conductor member 30, terminal 34 is connected to a variably selectable position along the helical length of resistance element 26 determined by the positioning of the contact 46 on such resistance element. In order to provide a somewhat stronger mechanical connection between the contact supporting member 44 and the driving screw thread of the conductor member 30, it will be desirable in some instances to provide a pair of contact members 48 for engagement with respective ones of adjacent turns of the conductor member 30.

Fig. 3 shows the two semi-cylindrical sleeves 22 and 2 4 with the outer casing and end caps, as well as the rotary shaft and contact assembly, removed. The two sleeves are substantially identical except for the apertures in sleeve 22, which apertures are so positioned with respect to the semi-helical grooves on the concave surface of the sleeve as to expose one or the other of the helices lying in such grooves. Apertures 50, 52 and 54 are aligned with portions of three different turns of the helical resistance element 26, and aperture 56 is aligned with a portion of a turnof the conductor member 30. Electrical connections may be made through each of these apertures in any suitable manner between the particular helix thus exposed and the respective terminals shown in Fig. 1. Thus, terminal 34 is connected to conductor member 36' through aperture 56, while terminals 36, 33 and 4d are connected to resistance element 26 through the respective apertures 50, 52 and 54.

As previously stated, the two sleeves 22 and 24 are substantially identical except for these four apertures. Thus, a significant saving in the cost of manufacture may be effected by the production of a single item corresponding to sleeve 24, with the apertures of sleeve 22 then being punched or the like in half of the items so produced. The grooved, concave face of sleeve 24 is shown in Fig. 4, and aside from the existance of the aforementioned apertures in sleeve 22, the pattern of Fig. 4 is also that of the concave face of sleeve 22. It will be observed that if the two identical semi-cylindrical sleeves are positioned adjacent each other to approximate a cylindrical sleeve with the-ends of the two sleeves in substantial alignment, the adjacent ends of respective grooves 28'and 32 are not in the proper alignment. In fact,

'4 with the ends of the two sleeves in alignment, the grooves on the concave surfaces thereof are out of alignment a distance corresponding to one half of the pitch of the grooves.

In order to provide proper alignment of the several grooves, the two sleeves are relatively displaced along the length of the centraYaxis of the helices by a distance corresponding to this one half of the pitch of the grooves. This is the'positioning showir'in Fig. 3, wherein sleeve 22 is offset to the left of sleeve 24 a distance equal to one half of the pitch of the grooves 28 and 32. The support paths for the helices are thus substantially helical, with interruptions twice per turn at the edges of the two semicylindrical sleeves.

One of the obvious advantages afforded by the split sleeve construction is the increased facility of assembly of the potentiometer. With this construction, the two semi-cylindrical sleeves 22 and 24 are merely placed around the two helices (the resistance element 26 and the conductor member 30,) with the several turns thereof fitting into respective grooves in the concave surfaces of the sleeves, and then the substantially cylindrical assembly is placed within the outer casing 10 in a sliding fit with the inner cylindrical surface thereof.

Moreover, the construction of the preferred form of the device of the present invention wherein each of the semi-cylindrical sleeves is somewhat less than a half cylinder affords several advantages over the split sleeve devices of the prior art. The prior art devices employ split sleeves which are each a half cylinder, with the adjacent edges of the two sleeves being contiguous when the potentiometer is assembled. In such a construction there is no space left for movement of the two sleeves toward each other to take up any slack which may exist between the sleeves and the helices enclosed thereby as a result of, for example, a lack of uniformity between turns of the radii of the helices. That is to say, if one or more turns of either helix are of somewhat smaller radius than the remaining turns thereof, such smaller turns will not lie firmlyin the grooves that are intended to support them, and the structure is as a result not mechanically stable. Such mechanical instability generally proves intolerable where the potentiometer is required to operate under environmental conditions which include vibration. In the construction of the preferred form of the 'p'resent'invention such difficulties are substantially eliminated by virtue of the natural resilience of the insulating sleeve and the freedom afforded such sleeve to move in a direction radial of the central axis of the helical elements. Additionally, the construction of the present invention wherein the two sleeves are each less than a half cylinder allows for thermal expansion and contraction of the several elements within the potentiometer casing without deleterious mechanical forces being applied thereto.

The fact that the insulating support sleeves of the preferred form of the present invention are open-ended and quite thin in comparison to those of the prior art devices is another advantageous feature, since this construction permits easy access to the internal parts of the potentiometer when the end caps of the outer casing are removed. In the prior art devices, the insulating sleeves are more generally in the form of enclosing cups or the like. In addition, lesss weight is introduced into the unit by the utilization of the thin sleeves of the present invention when compared to the weight of the comparatively bulky insulating means of the prior art devices.

With further reference to the device of the prior art, even where a split sleeve construction is employed the two units or sleeves are generally not identical, but are complementary so as to afford matching of the several ends of the respective gropves at the adjacent edges of the two sleeves while having the ends of the sleeves in substantial alignment. Suchbonstruction necessitates the manufacture of two different sleeves, increasing the cost thereof. As stated in connection with the construction shown in Fig. 3, the device of the present invention has the advantage that the sleeves are substantially identical, with the desired matching of the groove ends being achieved by means of the half turn offset between the two sleeves.

The method of assembly of the potentiometer shown in Fig. 1 is readily apparent. The resistance element 26 and the conductor member 30 are first interdigitated and coaxially aligned so that each turn of the resistance element 26 lies between a respective pair of turns of the conductor member 30. The two sleeves 22 and 24 are then placed over the helices in diametrically opposed relationship, with sleeve 22 being offset relative to sleeve 24 a half pitch of the grooves as shown in Fig. 3. The sleeves are so positioned over the helices that resistance element 26 lies in the grooves 28, while the conductor member 30 lies in the grooves 32.

' As shown in Fig. 4, the grooves 28 and 32 substantially cover the concave surface of the sleeve 24, with each of the four edges of the sleeve being intersected by at least one of the grooves. This not only affords a manufacturing advantage in that the sleeves may be made in a long tubular form and then cut to the desired axial length, but small adjustments may be made rotationally in the positions of the two helices in order to position same at desired locations with respect to the apertures in sleeve 22 .to ensure that portions of the helices appear at the apertures therein. This is particularly important with respect to the apertures 52 and 56 which are near the ends of the sleeve 22; a small amount of rotation of either helix about the central axis thereof may bring an end thereof under the respective aperture where no such end existed without such minor adjustment.

Once the helices and the sleeves are positioned within the casing 10, connections are made between the respective helices and the several terminals shown in Fig. 1, as by soldering or the like. The rotary shaft 20 is then inserted within the casing, along with the contact assembly mounted thereon, and the end caps 12 and 14 are secured to the opposite ends of the casing to close the same and to support the shaft 20 for rotation.

In the operation of the potentiometer of the present invention, especially with respect to its operation as an electrical potential divider, the input potential is applied to the terminals 36, 38, and a variably tapped output potential is taken from terminal 34, the latter being connected to the slip-ring or conductor member 30, which, in turn, is connected at a variably selectable point to a portion of a turn of the helical resistance element 26. As the rotary shaft 20 is rotated in one direction, the contact 48 rides along the helical conductor member 30, and in being rotated about the shaft 20 in this manner it causes linear translation of the contact supporting member 44 along the length of the shaft 20 in one direction, with opposite rotation of the shaft resulting in translation of the contacts in the other direction along the shaft. Terminal 40 is provided as a fixed tap on the resistance element 26, where it is desired to tap off a fixed portion of the potential applied between the input terminals 36 and 38.

The invention has been described above in considerable detail, and particularly with reference to its application to electrical potentiometers having both a helical resistance element and a helical conductor member which serves as a slip-ring for the movable contacts. However, it will be apparent to those skilled in the art that the invention is also applicable to potentiometers which employ slip-ring means other than that shown, and wherein the only helical element is the resistance element itself. Further, the invention is also applicable to other helical units, including helical inductances which are adapted to be variably tapped in accordance with the setting of a rotary shaft or the like. Hence, the invention is not to be considered as limited to the particular details given, nor to the specific perpendicular 6 application towhich reference has been made during the description of the device, except insofar as may be required by the scope of the appended claims.

What is claimed is:

1. An electrical potentiometer comprising a helical re-' sistance element having a central axis, a rotary shaft lying on said axis, a contact supporting member mounted on said shaft for rotation therewith and for sliding motion relative thereto along the length thereof, an electrical contact mounted on said contact supporting member and slidably engaging a variably selectable portion of a single turn of said helical resistance element, a pair of terminals connected to said resistance element at spaced-apart points thereon, a terminal connected to said electrical contact, a substantially cylindrical casing surrounding said helical resistance element, and. a pair of semi-cylindrical insulat-' ing sleeves contiguous with the inner surface of said cylindrical casing and disposed in diametrically opposed rela tionship about said central axis, the concave surfaces of each of said semi-cylindrical sleeves having a plurality of semi-helical grooves therein each supporting a portion of a turn of said helical resistance element, said semicylindrical sleeves being substantially identical as to the number, pitch and positioning of said grooves, each of the four edges of the concave surfaces of each of said sleeves being intersected by at least one of the grooves on said surface, said sleeves being relatively displaced along said axis a distance equal to one half of the pitch of the grooves thereon, whereby the ends of the grooves in one of said sleeves are substantially aligned with corresponding ends of respective grooves in the other of said sleeves to provide a substantially helical support path for said helical resistance element.

2. An electrical potentiometer in accordance with claim 1, wherein the intersections between the cylindrical surfaces of each of said semi-cylindrical sleeves and a plane to said central axis are each significantly less than a semicircle, the said portions of said helical resistance element supported by the several grooves each being correspondingly less than a half turn of said helical resistance element, whereby the support path for said helical resistance element is an interrupted helix.

3. A method of assembling an electrical potentiometer of the type having a helical resistance element with a central axis and a rotary shaft on said axis for carrying an electrical contact in helical resistance element, comprising the steps of positioning corresponding portions of the turns of such helical resistance element in respective semi-helical grooves in the concave surface of a semi-cylindrical insulating sleeve,

positioning opposite corresponding portions of said helical resistance element in respective semi-helical grooves in the concave surface of a substantially identical semicylindrical insulating sleeve by displacing said substantially identical sleeve along said central axis the distance of one half the pitch of said helical resistance element,

sliding the substantially cylindrical assembly thus formed into a cylindrical casing the inner surface of which is contiguous with the convex surfaces of said insulating sleeves, placing a rotary shaft within said cylindrical casing along said central axis with a contact member engaging said helical resistance element and mounted on said shaft for rotation therewith and for sliding motion relative thereto along the length thereof, providing terminals in contact with said helical resistance element at spacedapart points thereon, providing a terminal in contact with said contact member, and closing both ends of said cylindrical casing with end caps each having means for supporting said shaft for rotation with respect thereto.

4. An electrical potentiometer comprising a helical conductor member having a central axis, a helical resistance element substantially coaxial with said helical conductor member with each of the turns thereof lying between a respective pair of adjacent turns of said helical conductor member, a rotary shaft lying on said axis, a

sliding engagement with said' agree contact supporting member mounted on said shaft for fot'atio'h therewith and for sliding motion relzlitive thereto alongthe length thereof, a pair of electrical conta'cts mounted on said contact'supporting member in mutually conducting relationship, one of said contacts slidably engaging" a variably selectable portion of one turn of said helical resistance element, theothercontact slidably engaging said conductor member, a pair of terminals con nected to said resistance element at spaced-apartpoints thereon, a terminal connected to said conductor member, a substantially cylindrical casing surrounding both said resistance element and said conductor member, and a pair of semi-cylindrical insulating sleeves contiguous with the inner surface of said cylindrical casing and disposed in diametrically opposed relationship about said central axis, the concave surfaces "of each of said semi-cylindrical sleeves having a plurality of semi-helical grooves therein, alternate ones of said grooves being adapted to support portions of respective turns of said resistance element, the remaining ones of said grooves being adapted to support portions of respective turns of said conductor member, said semi-cylindrical sleevesbeing substantially identical as to the number, pitch and positioning of said grooves, each of the four edges of the concave surfaces of each of said sleeves being intersected by at least one of the grooves onsaid surface, said sleeves being relatively displaced along said axis a distance equal to one half the pitch of the grooves thereon, whereby the ends of the grooves in one of said sleeves are substantially aligned With corresponding ends of respective grooves in the other of said sleeves to provide substantially helical support paths for both said resistance element and said conductor member. l V V 5. An electrical potentiometer in accordance with claim 4, wherein the intersections between the cylindrical surfaces of each of said semi-cylindrical sleeves and a plane perpendicular to said central axis are each significantly less than a semicircle, the said portions of said helical re sistance element and said helical conductor member supported by the several grooves each being correspondingly less than a half turn thereof, whereby the support paths for said helical resistance element and said helical conductor member are each interrupted helices.

6. In an electrical potentiometer of the type having a helical resistance element with a central axis and a rotary shaft on such axis for carrying an electrical contact in sliding engagement with said helical resistance element, an improved insulating support for the turns of such helical resistance element comprising a pair of semicylindrical sleeves the concave surfaces of each of which has a plurality of semi-helical grooves therein each supporting a portion of a turn of such helical resistance element, said semi-cylindrical sleeves being substantially identical as to the number, pitch and positioning of said grooves, each of the four edges of the concave surfaces of each of said sleeves being intersected by at least one of the grooves on said surface, said sleeves being disposed 1n diametrically opposed relationship about spch centrgl axis and being relatively displaced along such ax's a distance equal to one'half of the pitch of the grooves on said concave surfaces, whereby the ends of the grooves inane of said sleeves are substantially aligned with corresponding ends of respective grooves in the other of said sleeves to provide a substantially helical support path for said helical resistance element.

7. An improved insulating support for electrical potentiometer resistance element turns in accordance with claim 6, wherein the intersections between the cylindrical surfaces of each of said semi-cylindrical sleeves and a plane perpendicular to such central axis are each significantly less than a semi-circle, the said portions of such helical resistance element supported by the several grooves each being correspondingly less than a half turn of such helical resistance element, whereby the support path for such helical resistance element is an interrupted helix.

8. An arrangement for insulatively mounting a helically formed electrical component within a casing, comprising a pair of identical semi-cylindrical insulating shells each provided on its inner surface with semihelical grooves, and a casing surrounding said shells and securing them in position with their grooved surfaces facing one another but otfset axially to define a helical componentv receiving groove throughout their common inner cylindrical face.

9. An insulating support for mounting a helical electrical component, comprising a pair of duplicate internally helically grooved insulating semi-cylindrical shells disposed facing one another and axially offset by one-half the helical pitch to define a single, effectively continuous, helical support groove on the inner surface of the composite assembly.

10. Apparatus for mounting a helically formed electrical component within a casing, comprising a pair of identical semicylindrical shells each provided on its inner surface with at least one semi-helical groove, and a casing surrounding said shells and securing them in position with their grooved surfaces facing one another but offset axially to define a helical component receiving groove throughout their common inner cylindrical face.

11. A support for mounting a helical electrical component, comprising a number greater than unity of duplicate internally helically grooved shells each comprising a portion of a cylinder, said shells being disposed in opposed relation about the axis of said helical electrical component and axially offset a distance equal to the pitch of said helical component divided by said number to derlne a single, effectively continuous, helical support groove on the inner surface of the composite assembly.

References Cited in the file of this patent UNITED STATES PATENTS 

